What are they?
Copepods fall under the classification of Crustacea and are reminiscent of very small one-eyed lobsters (measuring less than 0.5 mm to 2.0 mm in length). They are a form of zooplankton that occurs naturally in many freshwater ecosystems (copepods can also exist in saltwater ecosystems). There is a wide variety of copepods that also have a wide diet, copepods can be found in the form of filterers/collectors/ or predators – feeding mostly on algae, bacteria and small particles of detritus. Some larger species are predatorial and will actively hunt and eat smaller zooplankton (such as soft-bodied rotifers).
Although copepods come in a range of sizes and shapes, they can usually be identified as copepods easily enough (distinguishing the exact species is another story). They tend to have a bullet-shaped body with extruding antennae and a (usually red) eyespot located in the middle of their head. Their segmented body is split into the head with is usually fused with the first thoracic segment, this is connected to the abdomen which then splits into two spiny tails (caudal rami). Females may be carrying eggs on their abdomen which appear as two large sacks just above the tail.
Copepods may also be identified through how they move in the water, they have a signature quick flicking movement, often occurring in small fast bursts.
Where do they exist?
Copepods can be found in just about any healthy body of water from ponds and lakes to rivers and marshes.
What role do they play in the ecosystem?
They play a vital role in aquatic food webs both as primary and secondary consumers, as well as a major source of food for many larger invertebrates and vertebrates (such as fish fry, and small species of fish). They form a link between primary producers and higher predators. Having 13 distinct life stages (egg, six naupliar stages, five copepodite stages, and adult) they have a range of sizes allowing them to be food for a large variety of small predators. They are also very important in keeping up water quality as they help in filtering out debris and decomposing matter.
Why put them into my Aquarium?
Copepods assist in improving the natural aquarium’s biodiversity and general tank health. Copepods existing in the water help to remove deteriorating plant matter as well as providing a live, nutritious and stimulating feed for smaller fish and fry. Freshwater copepods are relatively easily cultivated once established and are good just to introduce into the ecosystem of your tank. Or for breeders, they are an excellent live food to cultivate for fry as an alternative (much more nutritious) to brine shrimp. (Having a higher protein content than brine shrimp copepods are shown to improve the health and growth rates of young fish.)
W.H. Kwok, K., Souissi, S. and Dur, G. (2015). Aquatic Ecotoxicology. 1st ed. Elsevier Inc.
James H. Thorp and Alan P. Covich 2019 Thorp and Covich’s Freshwater InvertebratesVolume 4: Keys to Palaearctic Fauna. [Accessed 18 Feb. 2020]
James H. Thorp and Alan P. Covich 2009 Ecology and Classification of North American Freshwater Invertebrates 3rd Edition [Accessed 18 Feb. 2020]